Select the active object ? with python
You can usually retrieve the active object with: bpy.context.active_object or bpy.context.object
If you need to take the view layer into account: bpy.context.view_layer.objects.active
Chances are your view layer is already set, but if the view layer needs to be set, you can first do: bpy.context.window.view_layer = view_layer
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To make sure the active object is also a selected object, you can do this:
import bpy
my_object = bpy.context.view_layer.objects.active # assign variable
my_object.select_set(True) # select the object

Setting an object as active is a similar process. See this answer to learn about that.
Many operations require both an active object and at least one additional selected object.